sexta-feira, 26 de abril de 2013

The answer is blowing in the wind

The warming went into the deep end

There has been an unusual surge of interest in the climate sensitivity based on the last decade’s worth of temperature measurements, and a lengthy story in the Economist
tries to argue that the climate sensitivity may be lower than
previously estimated. I think its conclusion is somewhat misguided
because it missed some important pieces of information (also see
skepticalscience’s take on this story here).

While the Economist referred to some unpublished work, it missed a new paper by Balmaseda et al. (2013)
which provides a more in-depth insight. Balmaseda et al suggest that
the recent years may not have much effect on the climate sensitivity
after all, and according to their analysis, it is the winds blowing over
the oceans that may be responsible for the ‘slow-down’ presented in the
Economist.
It is well-known that changes in temperature on decadal time scales
are strongly influenced by natural and internal variations, and should
not be confused with a long-term trend (Easterling and Wehner, 2009;Foster and Rahmstorf, 2011).
An intensification of the trades has affected surface ocean currents called the subtropical gyres, and these changes have resulted in a predominance of the La Nina state. The La Nina phase is associated with a lower global mean temperature than usual.
Balmaseda et al’s results also suggested that a negative phase of the pacific decadal oscillation
(PDO) may have made an imprint on the most recent years. In addition,
they found that the deep ocean has warmed over the recent years, while
the upper 300m of the oceans have ‘stabilised’.
The oceans can be compared to a battery that needs to be recharged
after going flat. After the powerful 1997-98 El Nino, heat flowed out of
the tropical oceans in order to heat the atmosphere (evaporative
cooling) and the higher latitudes. The warming resumed after the
‘deflation’, but something happened after 1998: since then, the warming
has involved the deep ocean to a much greater extent. A weakening of the
Atlantic meridional overturning circulation (MOC) may have played a role in the deep ocean warming.
The recent changes in these decade-scale variations appear to have masked the real accumulation of heat on Earth.
The new knowledge from this paper, the way I read it, is the
revelation of the role of winds for vertical mixing/diffusion of heat in
a new analysis of the world oceans. Their results were derived through a
set of different experiments testing the sensitivity to various
assumptions and choices made for data inclusion and the ocean model
assimilation set-up.
The analysis involved a brand new ocean analysis (ORAS4; Balmaseda et al., 2013) based on an optimal use of observations, data assimilation, and an ocean model forced with state-of-the-art description of the atmosphere (reanalyses).
By running a set of different experiments with the ocean model,
including different conditions, such as surface winds and different
types of data, they explored which influence the different conditions
have on their final conclusion.
The finding that the winds play a role for the state of the warming
may not be surprising to oceanographers, although it may not necessarily
be the first thing a meteorologist may consider.